Słomkowski's technical musings

Playing with software, hardware and touching the sky with a paraglider.

Paramotor helmet with headset


Skateboard helmet accompanied with Peltor earmuffs and communication equipment is a legitimate alternative to expensive off-the-shelf paramotor helmets which in fact are built on the same principle. I use military-grade differential dynamic microphone to achieve superior speech intelligibility.

Since building things is fun, I built myself a paragliding helmet, instead of buying one. I saved some money too! I used generic skateboard helmet with Peltor earmuffs fitted with communication equipment. I was heavily inspired by the helmet made by SP3OTZ (website in Polish only). I have taken plenty of photos so the construction details are clearly visible.

The headset works with Baofeng UV-5R and my Kobo BlueFly variometer. After almost two years of intensive use I can tell that receiving voice quality and noise muffling are excellent. The modulation is loud and clear, but at full throttle it becomes almost unintelligible because of noise generated by ignition. This might be fixed in the future by trying to shield ignition cable in my paramotor.

To make high quality communication set you need high quality speakers, microphones and mechanical components. The best source of them are military surplus shops. They have plenty of military and aviation headsets at cheap prices which also have rugged microphone arms.

Update April 2020: the article now includes new microphone circuit schematic and extended information about voice transducers. Photos have been remade to include the changes.

Integrating voice equipment into Peltor earmuffs

The base for the headset consists of a pair of Peltor Optime II ear defenders with attachments suitable for hard hat mount. They are quite spacey inside and, according to manufacturer, reduce noise by 31 dB, which is more than enough. If you need even more muffling and space for internals, try Peltor Optime III.

Push-To-Talk button is located on the right earmuff. I have throttle in my left hand, so it is natural to press the PTT button by the other hand. The cable and the microphone arm is attached to the opposite earmuff. Two of them are connected by four-wire cable, which goes in the groove etched in the helmet’s inner shell, around the head. Two wires are used by PTT button, the other two - by the right earphone. I used cable grommets so the cables are protected from vibrations and look quite professional.

The stock earmuff contains soft foam. I cut and removed a part of the foam to accommodate the transducers. To fix the earphone in place I used a piece of hard foam cut in the shape of the earphone.

Earphone transducer

Good transducer for paramotoring should be rugged and properly reproduce the spectrum of human voice, damping other frequencies. I bought a pair of military-grade speakers Racal 782-5393, commonly used in Clansman radio equipment. Not much information is available, I found the following:

Parameter: Value:
DMC Z42
P/NO 27187-11
Impedance 300 Ω

EM-1 and EM-2 headphones are extremely similar to my headpiece, but mine has different painting: green with red flash.

Microphone

The microphone I used was Racal Acoustics 13750, also known as 5965-99-911-8230, which is a differential microphone, designed to be used in noisy environment. It’s similar to Russian DMSZ1. I bought it at military surplus store, along with the speakers. I haven’t managed to find the datasheet, however I collected all useful information from Eylex tank helmet brochure, which contains parameters of Racal 13750 and other military microphones. The device overview:

Parameter: Value:
Part NO. 13750
NSN NO. 5965-99-911-8230
Transducer Type tropicalised noise cancelling magnetic microphone
Sensitivity -65 dB re 1 V/Pa at 1kHz with sound source 9.5 mm distant from front of microphone. The peak output is approx. 3 mV for loud speech close to the microphone.
Impedance 300 Ω ± 20% at 1 kHz.
Noise cancellation Approx. 35 dB at 100 Hz gradually reducing to 0 dB at 1.2 kHz. Other spec says: approximately 20 dB at low frequencies, reducing in effect as the frequency increases and reverting to normal pressure operation at 3.2 kHz.

… and its frequency response chart (output in dBV/Pa):

Characteristics of Racal 13750 microphone. Output in dB re 1 V/Pa.

Microphone amplifier module and wiring

Racal mike has comparably low sensitivity (-65 dBV/Pa), most analog electret and MEMS microphones have sensitivity between –46 dBV/Pa and –35 dBV/Pa. Signal from the microphone is too weak to drive Baofeng directly, an amplifier with gain 15 to 20 dB is essential.

At first I tested integrated amplifier SSM2167, which has also compressor built-in, but its input noise gate has too little sensitivity (approx. -55 dBV) to open reliably, when I am talking. This results in heavily distorted speech.

Finally, I used MAX9812L module, widely available on AliExpress or eBay. In fact, there are several modules available, shown in the photos:

Bare MAX9812L handles 2.7 - 3.6 V. First two modules contain voltage stabilizer XC6206, which allows powering them from 3 - 5 V supply. Both of them have more or less identical schematics, each of them being basic application of MAX9812L with low drop stabiliser:

Schematic of the #2 module. Basic application of MAX9812L and XC6206.

Off-the-shelf module acquired from AliExpress was modified in following manner, with process illustrated by the photos:

The complete schematic of the headset:

Electrical wiring schematic of the helmet.
Electrical wiring schematic of the helmet.
↑ click to enlarge ↑

Headset is powered through multi-wire headphone cable, which has shielding on microphone line. At the end of the cable, two plugs are fixed: one goes to Baofeng radio, the other to the variometer. Baofeng uses Kenwood-compatible composite plug which consists of of 2.5 mm and 3.5 mm jack plugs spaced 12 mm apart. Speaker signals from both devices go to the speakers through protection resistors R1, R3. C8 is intended to reduce noise from the ignition system. In my particular circuit, variometer has optimal volume when R1 = 47 Ω.

Great consideration was put into reducing RF and ignition noise. Antenna is in close proximity to the headphone cable, so it catches significant part of RF energy. I put capacitors and chokes to reduce the effect of RF power on the MAX9812L. The other cable, the one from the microphone to the amplifier, has to be shielded, preferably be made of two wires with external shielding - the shield is grounded at amplifier’s side. In other places shielding is not critical, but I wrapped the whole PCB in copper adhesive tape nonetheless. A ferrite core extracted from computer PSU was put on the main cable where it enters the shell, hopefully reducing the noise.

The amplifier itself is basic application of MAX9812L enhanced with RF-blocking capacitors of 1 nF: C1, C5 and 4.7 μH chokes: L1 and L2. Without them, my circuit was blocked by RF and silenced. R2 pulls the radio’s input bias voltage to ~2 V; without it, the input stays at 3.3 V, which causes speech distortion.

This circuit requires modification of Baofeng radio to work, because microphone amplifier requires constant 3.3 V power. The easy mod is described in other article.

Sourcing the cable and Kenwood plug

The most problematic part to find is Kenwood plug, which consists of two jacks: 2.5 mm and 3.5 mm, spaced 12 mm apart. The best course of action to obtain it is to buy handheld mic for Baofeng, since they are dirt cheap. Majority of them use 4-wire cables, but we need five wires! 5-wire cables are used in dual-PTT microphones. The microphone which has dual Push-To-Talk and uses five wires is shown in the photo:

For reference, I include cable colors in this particular microphone:

Color: Connector:
green 2.5 tip
red 3.5 ring
yellow 3.5 sleeve
black 3.5 tip
blue 2.5 sleeve

My cable is made of three pieces of cabling: high quality headset cable with shielded wires ~70 cm, Baofeng plug with short piece of cable and ~70 cm of shielded audio wire terminated with right-angled mini jack plug, which goes to the vario. Ferrite bead is placed on the cable’s end inside the earmuff.

Old electrical wiring

From 2016 to 2019 I was using one-transistor microphone amplifier, however it was too quiet. I provide the old schematic for reference. It might suit you if your microphone has better efficiency than mine.

Old electrical wiring schematic.
Old electrical wiring schematic.
↑ click to enlarge ↑

Modifying the stock skateboard helmet

Warning! The manufacturer forbids modification of the helmet because it might lower its protection capabilities. You have been warned!

The helmet is a basic skateboarding helmet of Oxelo brand, bought at Decathlon. A piece of outer shell with styrofoam underneath was cut to accommodate the earmuffs. The exact change is visible when you compare my helmet with vanilla one:

I followed the advice of SP3OTZ. Cutting procedure is as follows: first I covered the side of the helmet with masking tape so it was easy to draw on it using pen. Then I put the helmet on in front of the mirror and determined the optimal position of the earmuffs. I marked spots for mounting holes and drew the shape which follows roughly the shape of the earmuff with play of 15 mm. The shape was mirrored on the other side. Then I cut the shell with the Dremel multi tool and polished the edges with sand paper: 150 then 400 grit.

I couldn’t use original helmet straps because of the colliding earmuffs. Instead, I fixed 10 mm hard polyethylene tubing to go round the earmuff. This solution is common in other paramotor helmets. It’s quite rigid so it goes round the earmuff nicely. I cut it to proper length end then flatten the ends after having them heated up by a heat gun. This made them soft and easy to flatten using clamps. Then I drilled 5 mm holes in the flattened ends and riveted them to the helmet’s outer shell.

The new strap is made from pieces of velcro strap salvaged from military headset. Despite being velcro it holds better than the original plastic clip strap.

Cable connecting the earmuffs goes inside the shell and is glued to the inner styrofoam. Where it goes outside, it is secured by 6 mm grommets. Eamuffs are fastened to the outer shell by M6 screws and nylon locknuts. It is important that the screws are as short as possible to avoid penetrating your skull during potential impact!

As finishing touch, I attached camera mount compatible with Chinese action cameras.